In the 16th and 17th centuries, before the invention of
commercial fertilizer, everything had to be transported by ship. Therefore,
large shipments of manure were common. It was shipped dry, because in dry
form it weighed a lot less than when wet, but once water (at sea) hit it, it
not only became heavier, but the process of fermentation began again, of
which a by-product is methane gas.

As the stuff was stored below decks in bundles you can see what could (and
did) happen. Methane began to build up below decks and the first time
someone came below at night with a lantern, BOOOOM!

Several ships were destroyed in this manner before it was determined just
what was happening. After that, the bundles of manure were always stamped
with the term "Ship High In Transit" on them, which meant for the sailors to
stow it high enough off the lower decks so that any water that came into the
hold would not touch this volatile cargo and start the production of
methane.

Thus evolved the term "S.H.I.T," which has come down through the centuries
and is in use to this very day.